This invention relates generally to the art of rotational molding. More specifically, this invention relates to a method of rotational molding a one-piece thin walled hollow article in combination with a gravity cast member, which has both its inner and outer surfaces defined by mold surfaces.
Rotational molding apparatus and processes for producing thin walled hollow articles are well known in the art. Even molding using the xe2x80x9cstop rotationxe2x80x9d process produces a thicker support base. These support bases, however, predominantly have only one surface defined by the mold wall. The typical rotational molding process starts with placing a predetermined amount of polymeric resin in an open mold. The mold is closed, placed in an oven and continually rotated about two axes. The plastic melts and flows over the heated inside surface of the mold, forming a generally even wall thickness. After sufficient processing time, the mold is removed from the oven and cooled while still being continuously rotated. The mold is then opened and the part removed. Typically a uniform wall thickness is desired for a rotationally molded part. It is critical in rotationally molding a part that the plastic have the necessary mold coating and flow characteristics to assure a uniform wall thickness. For this reason, great attention is paid to the formulation of special compositions of molding powders and liquids to provide high flow and uniform coating of the mold surfaces. Additional process parameters, such as the heat transfer through a selected portion of the mold and the rotational speed ratio between the two axes are varied to achieve a uniform wall thickness. Nevertheless, it can be difficult to assure uniform coating of a thin walled part because effects of surface adhesion prevent even bulk fluid flow given the small volume of plastic attempting to spread over a large surface.
Within limits, some variation of wall thickness can be achieved when desired. Taking advantage of the characteristic that the portion of the mold which reaches curing temperature first will accumulate the thickest layer of plastic, selective variation of heat transfer can induce thicker or thinner walls in a given portion of the part. This is achieved by adjusting the mold thickness or material to vary heat conductivity in select regions of the mold and by varying the heat input to the mold by preheating or shielding.
Another method of achieving variation of wall thickness is by adjusting the rotation method. As mentioned above, changing the ratio of turns about one axis relative to another can adjust the wall thickness, within limits based on the part and rotation axis geometry. One variation of this technique is found in U.S. Pat. No. 4,292,015, which describes an apparatus for rotational molding generally known as a rock and roll machine. These machines are well suited for moldings with very long length to diameter ratios such as canoes and kayaks. The mold, mounted on a cradle, is rocked back and forth on a stationary, transverse horizontal axis while it is rotated about a longitudinal moving axis perpendicular to the horizontal axis. In this type of machine the mold is not rotated end for end. The ""015 patent teaches variation of the wall thickness due to the ability to vary the speed of the tilting action about the horizontal axis during the tilting. By increasing the dwell, for example at the horizontal position, more plastic will accumulate at the central portion of the article. Rotation is continued about the longitudinal axis and the wall thickness is substantially even about the sections along the longitudinal axis.
An additional variation of the rotational molding process is referred to as xe2x80x9cstop-rotationxe2x80x9d. This process utilizes a liquid polymeric compound, which reacts to heat in a somewhat similar manner as that described above. The mold rotation, however, is stopped on at least one axis during the heating process prior to fusion of the material occurring. This stopping of the mold rotation while the compound is still liquid, allows most of the liquid polymer to drain off of the upper mold surfaces and accumulate in the lower regions of the mold. The inner surface of this accumulated material is self-leveling, as it is still in a liquid state, and has no mold surface in contact with it.
It is apparent that none of the methods described above are suited to the desire of making the one-piece article of this invention.
Accordingly, this invention provides a rotational mold structure which can be developed according to this invention so that a desired inflatable toroidal shaped section, with a support base and an integral cover or dome is produced, molded all in one piece. This article overcomes the problems and disadvantages of the conventional techniques in the art, which required multiple pieces separately made and later put together.
Briefly, the invention includes a one-piece face mask with a dome shape outer wall terminating at its circumference in a thickened base and an inflatable cushion of toroidal shape extending in a direction away from said dome and said base, the cushion being integral with said base and having a hollow chamber.